That short stretch of "no man's land" between the mouth of the chamber and the beginning of the rifling is the throat. It can vary in length and diameter depending on the manufacturer.
The leade comes into play here also. Leade is the "butt end" of the rifling lands. It can be sharp and abrupt or slightly tapered for a short (or long) distance until full height of the lands is reached. (I have a special .30 throat reamer that creates a leade with a 1 1/2 degree included angle taper. It is an accuracy enhancer, but the downside is that it then demands that cast bullets be seated a lot further out. Sometimes you just can't have your cake and eat it too.)
A cast bullet must be started straight into the rifling if best accuracy is the goal. How well the bullet fits the throat is key to that, and the leade can (should) help guide the bullet in also. Guys who are anal about cast bullet accuracy pay strict attention to all that and often go so far as to select the bullet size and shape that best fits the configuration of their throat/leade, and seat it accordingly so that it's guided straight and true with a minimum of distortion. Other guys reconfigure their throat/leade configuration to accomodate a specific bullet. Cast bullets are soft fragile little beasties that don't cotton to the kind of abuse that jacketed ones can shrug off as they start their long journey down the tube.
I suspect you're loading to magazine-compatible overall length? That may or may not be optimal. In fact it probably isn't. Seat it out until the nose is engraved by the leade, even if perchance it's only sticking inside the case neck by its toenails and try that. If rapid fire from the magazine is your goal, or a magazine full in reserve for hunting is of the essence, then you likely will have to make a compromise in your accuracy goals. Or maybe not. That's why you have to systematically eliminate all these variables to find out.
A well fitted bullet will shoot pretty well across a relatively wide range of velocities, but like any other load/rifle combo there'll be a sweet spot. An old gnarled cast bullet veteran will find that sweet spot pretty quickly, but only because he put on his thinking cap and analyzed his equipment's requirements before he casts the first bullet and seats it in a case.
This is the beauty of the cast bullet hobby. It can be approached on many levels, but all approaches must be thought about. It requires a little more participation of the shooter's brain. It can be richly rewarding or supremely frustrating- whichever one chooses to make it.
Also thank you for all the info you been providing.
